Like with title revocation, vassals may refuse by rising in rebellion, unless you hold them as prisoners. Retracting a vassal results in a opinion penalty of -45 if the retracted vassal is de jure part of the target's titles and -20 otherwise. If the ruler did not have a reason, other direct vassals will have opinion penalties of -10 (de jure) or -5. Valid reasons are:

Opinion modifiers such as "Traitor" can be used to retract one vassal (instead of revoking one title)

Imperial Administration and Iqta Government allow free retraction of vassals

Revoking titles is a ruler's primary way of dealing with vassals who have gotten too powerful. Vassals who have acquired multiple landed titles can be brought back in line by the ruler getting claims on their titles and revoking them. It also allows getting rid of troublesome vassals who dislike you or whose line of succession would pass the title out of the realm. It can also be used to simply replace vassals with more desirable ones, such as members of your dynasty.

Vassals who rebel when you try to revoke their titles provide the opportunity to revoke two titles without incurring tyranny. Once you've crushed the revolt, revoke a title you don't have a claim on without incurring tyranny because they are a traitor, then revoke the title you do have a claim on. Because the rebels will be imprisoned after losing the war, the opinion penalties are not a problem. Put them in house arrest to retain the benefits of a vassal who can't cause trouble for as long as possible, or throw them in the oubliette so they'll quickly succeed to an heir who doesn't care that you revoked a title.